OverviewA non-religious mother, Chris, and her young daughter, Regan, lead normal lives until Regan comes down with unusual behavior. Thinking it is medically related, Chris takes her daughter in for extensive tests. As her daughter's aggression and strength increases, her mother turns to alternative treatments. After a mysterious death from Regan's bedroom window, the police and a priest investigate the scene. Chris is convinced her daughter is involved and asks the priest to intervene. Bringing in another priest, named Merrin, the two men evaluate Regan's condition. Merrin is aware of an evil entity that wants to return to modern times through his trips to archaeological sites. He believes that Regan is possessed by this demon. Between the two priests and the mother, Regan must be exorcised to remove the demon before it is too late.